Don't panic if a queue exists, but is empty when repairing metadata.

service/src/test/java/org/whispersystems/textsecuregcm/storage/MessagesCacheTest.java

@@ -120,6 +120,12 @@ public class MessagesCacheTest extends AbstractRedisClusterTest {
         assertTrue(messagesCache.getMessagesToPersist(DESTINATION_UUID, DESTINATION_DEVICE_ID, 100).isEmpty());
     }
 
+    @Test
+    public void testRepairEmptyQueueMetadata() {
+        // We're happy as long as this doesn't throw an exception
+        messagesCache.repairMetadata(DESTINATION_UUID, DESTINATION_DEVICE_ID);
+    }
+
     @Test
     @Parameters({"true", "false"})
     public void testRemoveById(final boolean sealedSender) {